The subject matter disclosed herein relates to a system for managing power flow between an alternate energy source and a storage device. More specifically, a first power converter is connected between the output of the alternate energy source and a load and a second power converter is connected between the output of the alternate energy source and the storage device, and operation of the power converters is controlled to provide consistent and predictable power to the load.
As is known to those skilled in the art, alternate energy sources often have a variable power generation capacity and generate a voltage having a variable amplitude and/or frequency. For example, a photovoltaic (PV) array generates a DC voltage and current having amplitudes that are a function of the light incident on the PV panel. As the angle of the sun varies in the sky or as clouds pass between the sun and a PV panel, the amount and intensity of light incident on the PV panel changes, thereby varying the energy generated by the PV panel. Similarly, a wind turbine rotates as a function of the wind speed passing over the blades of the wind turbine. The alternator driven by the rotating blades, therefore, generates a voltage having a variable frequency and/or amplitude as a function of the wind speed.
It is desirable to convert the variable voltage generated by the alternate energy sources to a voltage having an amplitude and/or frequency that remains generally constant or within an acceptable range of variation in order to provide the voltage to a load or back to the utility power grid. Historically, it has been known to provide a first power converter between the output of the alternate energy source and the load or the utility grid in order to convert the variable power generated by the alternate energy source to a constant power that may be provided to a load or to the utility power grid.
Due to the variable nature of energy generation, there may be periods of time when the alternate energy source generates more energy than required by loads connected to the system. It is desirable to provide energy storage to capture the energy generated during these periods. The energy stored during periods of excess generating capacity may subsequently be provided to the loads during periods in which the generating capacity is less than the energy required by the loads. Historically, a second power converter has been provided to regulate energy transfer to and from the energy storage device. During periods of excess energy generation when power is being transferred to the energy storage device, the energy storage device and power converter operate as a load to the alternate energy source. Similar to other loads, it is desirable to connect the power converter to an energy source delivering a constant voltage. The second power converter is, therefore, typically connected at the output of the first power converter.
As is understood in the art, the first power converter, may utilize a maximum power point tracking (MPPT) routine to control the power transfer from the alternate energy source to the load. The MPPT routine controls the rate at which energy is transferred from the alternate energy source to the load, and, as the name implies, keeps the first power converter operating at a point where the first power converter transfers power at a maximum efficiency as a function of the amount of power being generated.
Operation of the first power converter under a MPPT routine, however, does not necessarily result in the maximum power available being transferred from the alternate energy source. Because the size and cost of power conversion devices increases as the power ratings of the devices increase and because an alternate energy generating source may not regularly generate at its peak capacity, it may not be cost effective to provide a power converter capable of transferring all of the energy generated when the alternate energy source is operating at its peak generation capacity. Providing a power converter capable of transferring the peak energy that may be output by the alternate energy source will result in the power converter regularly operating at less than its rated capacity. This excess capacity of the power converter results in a more expensive power converter, which also requires more space for installation. Thus, it may be more economical to provide a power converter between the output of the alternate energy source and the load that is rated at less than the peak capacity of the alternate energy source. The cost and size required for installation of the power converter may be reduced while allowing the power converter to more regularly operate at its rated capacity.
However, because the power converter is rated at less than the maximum generation capacity of the alternate energy source, periods of time exist during which some of the capacity of the alternate energy source is lost. Thus, it would be desirable to provide a system that efficiently captures and utilizes the maximum generation capacity of the alternate energy source.